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Air Conditioners and 1st law

  1. Sep 6, 2003 #1
    Can you use the heat energy extracted by an A/C to do some useful work? Or does this somehow violate the 1st law in such a way to negate energy conservation....doesn't appear so; of course, the heat energy coming out into the ambient temp would be small, nil, and thus the kind of work you could get out would be poor quality.

  2. jcsd
  3. Sep 6, 2003 #2
    There is nothing to prevent you
    from using the heat extracted by
    an AC unit to do work. The ideal
    power available will be in
    proportion to the difference in
    temperature between the heat ex-
    tracted and the ambient temp of
    the environment you put it to
    work in. In the average case this
    will hardly be nil. There would
    be enough energy to make practical
    use of if you're not too ambit-
  4. Sep 6, 2003 #3


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    Yes. No. If you want to "drive" a thermopile with the temperature difference established by the A/C, there's no problem --- do you accomplish anything? No.
  5. Sep 6, 2003 #4
    Thanks, Zoobyshoe (grt handle, btw).

    Ok, maybe I shouldn't have said nil, but I think the efficiency of this type of heat engine is less than for instance in the case for internal combustion.

    So, the problem would come down to collecting (maybe not so big, since most A/C's have nice heat exhcanger fins), and then with the relatively low temp, what could you do with it?

    I mean, if this was a relatively simple thing, wouldn't we have expected to see this as more standard equipment on a/c's for industry? (since they use more kilowatts for their pumps), or even on homes?

    One reason I thought about this, is we are considering adding solar power to our home, and it was pointed out, given our location - sunny southern ca, and if we have enough panels, that the net input at times *could* possibly be greater than consumption, and that this extra amt. can be fed back into the grid, reducing our energy costs. I was simply wondering if there was a similiar way to reduce the cost of a/c this way, by making use of the wasted thermal energy...
  6. Sep 6, 2003 #5
    Wow, it's funny that you should mention this. I had totally fogotten about that, but some people from the Physical Plant at my university gave a presentation about energy stuff.

    One of the guys talked about how he used the heat from his air conditioning. It was used to heat water. It had a very significant impact on the amount of electricity that he had to purchase to heat water.

    Someone mentioned that it probably wasn't feasable, based on the evidence that it isn't something that you see. The sad truth is that once a technology becomes well-established, change tends to become very unlikely. I wouldn't doubt that we live in the most energy-inefficient country in the world.
  7. Sep 6, 2003 #6
    In practical terms the best use
    that might be made of the heat
    from an AC unit would be to rig
    it up to work in conjunction with
    your water heater, to heat, or at
    least preheat water from the cold
    water mains.

    This would best be accomplished at
    the manufacturing level with all
    the proper engineering. A person
    would have to be a serious tinker-
    er to spend the time and energy to
    retrofit an AC unit to do this.
    There is the "contraption" factor
    to consider: would it end up being
    something that detracted from the
    appearance of your home?
    The energy, however, is there and

    Since you live here in Sunny So.
    Cal. it would actually be easier
    to install solar water heating
    panels to cut down on your water
    heating bill. This would require
    plumbing them into your water
    heater, (and might require permits
    for all I know). It is something
    you'd have to research to see if
    it is worth it in the long run.

  8. Sep 6, 2003 #7
    Whoa! Dissident Dan! You posted
    while I was composing mine! Spooky
    mental wavelength matchup!
  9. Sep 6, 2003 #8

    I was more curious about this from the standpoint of thermodynamics, and assure you, I'm not nearly nutty enough to try this at home!

    I'm taking a biophysics class at the moment, and we just revisited gibbs free energy and the role of 'free energy transducers', such that living organisms apparnelty play..got me to thinkin 'bout thermo again (took it many, many years ago as an undergraduate..and I'm RUSTY!!
  10. Sep 6, 2003 #9
  11. Sep 7, 2003 #10
    In this case let me just say that
    there is probably more energy in
    this "waste" heat than you susp-
    ected at first, and, as I said,
    if you not too ambitious, it
    could be put to some practical

  12. Sep 7, 2003 #11
    Agreed, Robin. Could this be the
    work of that dastardly fiend, Fz+
    and his Neo-Quantum thought inser-
    tion ray gun? To the Batmobile!
  13. Sep 7, 2003 #12
    Interesting idea. I imagine the only thing you could do with the heat is heat water. Some how incorporate that heated water into a system with the water heater. Surprisingly that sounds like a practical idea, could save quite a bit of energy over a few months of use during the summer. But if you're truly conscious about conserving energy, why not just use solar power. ;)
  14. Sep 7, 2003 #13
    I bet you could roast a Turkey.
  15. Sep 7, 2003 #14


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    You guys are treating this idea well, but this sort of thing is where a lot of free energy claims come from. People often make the mistake of thinking they can recover enough waste heat from a cycle to run the cycle - a direct violation of the 1st law. Thats what a self-powered water wheel does.

    Yeah, you could use your A/C to heat water but I'm not sure how practical it would be. Probably wouldn't do much good.
  16. Sep 7, 2003 #15


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    The only limitation from thermodynamics is that you couldn't recover enough energy from the waste heat to run the airconditioner that produced it. That would be perpetual motion.

    The trouble with using it is that devices that work off small heat differences tend to be huge and slow. As the folks who worked with those old solar hot water systems found.
  17. Sep 7, 2003 #16
    What this thread is talking about is exactly what a water source heat pump loop is designed to do. The explanation is a little long, but here goes.

    A water source heat pump is basically an Air Conditioning unit with a fan, a direct expansion coil, a compressor a condensor heat exchanger and a reversing valve to change the direction of refrigerant from evaporator coil to condensing heat exchanger when shifting from cooling to heating. Heat pumps cool in the warm weather and heat in the cold. The water source loop takes the rejected heat from the unit and cools the refrigerant.

    With that in mind, imagine a building with a lot of outside walls and a bunch of inside rooms with a lot of heat load like computers. The heap pimps on the outside of the building might have to heat in the morning because of the cold air outside so they pull heat from the water loop. At the same time the rooms on the inside of the building may be hot because of the computers putting heat into the room. The heap pump takes that heat out of the air and adds the heat to the water loop raising the loop temperature back up.

    In theory, this kind of system balances well in the cold weather and no heat has to be added to the water or removed. In the warm weather the whole building is hot and needs to be cooled so the water loop temperature rises and must be rejected to the air through a cooling tower or to another heat sink such as the ground in geothermal or the water in a water to water heat exchanger. But in the cold weather, there is a savings of energy because they don't have to heat the water loop as it self balances.
  18. Sep 7, 2003 #17
    Well, you usually can't get all the heat that most USAmericans desire just from a passive solar system, unless maybe you had a really large tank. You could use this in conjunction with the passive solar.

    Also, with this system, you could keep all the working behind walls, which may have an aesthetic component that some people desire.
  19. Sep 7, 2003 #18
    I forgot another related item oftem placed in homes with geothermal heat pumps is the preheater for domestic hot water. This uses a heat exchanger to preheat the water going to the water heater and furhter cool the water going to the AC units. Even if it can't cover the heater load, it can contribute and also reduce the thermal shock on the water heater.
  20. Sep 8, 2003 #19


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    You need the 2nd law to understand what can be done with the heat energy. It tells you that if you want to use the heat rejected by an a/c unit (and that unit is already reasonably efficient), the unit will then be rejecting its heat into a reservoir of slightly higher temperature. This requires the unit to use more energy. This extra energy will always be greater than the mechanical energy you can get out of the rejected heat.

    In symbols, let's say the a/c unit is cooling the reservoir at temp. T1, and dumping heat into ambient outside temp. T2. It is also applying work W1. To use some of this heat, the a/c unit will effectively be dumping heat into a reservoir of temp. T3, where T3>T2. To do this, W1 increases to W2. Now you can run a heat engine between T3 and T2 to produce work W3. But always W3<W2-W1, so nothing is gained.
  21. Sep 8, 2003 #20
    the unit will then be rejecting its heat into a reservoir of slightly higher temperature.
    Where do you get this?
  22. Sep 8, 2003 #21

    Ivan Seeking

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    This would normally be true since the exit temp of the heat exchanger [A/C condenser to water preheater] would be well above the ambient temp.
  23. Sep 8, 2003 #22
    In order to expell heat into the
    ambient temperature the AC unit
    it required to concentrate
    the heat to a temperature higher
    than the ambient temperture via
    work. It is going to do this
    whether or not you use this higher
    than ambient temperature heat to
    do something else.

    If instead of letting it flow into
    the ambient atmosphere you expose
    it to water coming cold from the
    main the heat will flow into that
    water and you have gained hotter
    water. This does not require more
    work from the compressor.

    I do not see where this notion of
    trying to put heat into a reser-
    voir of slightly higher temperat-
    ure came from.
  24. Sep 8, 2003 #23
    Not if other units are sharing the reservoir and removing the excess heat for heating operations elsewhere, or if the system is storing the heat for future use.

    Here is a quote from the Trane Applications Engineering Manual for Water Source Heat Pump System Design. Pg 17. "Energy cost savings can be accrued by recovering heat from interior zones for use near the building perimeter."

    Also, if you throw in a storage tank to store large quantities of low temperature water in the evenings you can bleed it into the system during the cooling operation of the entire building, then during the cooling operation heat is placed into the tank to offset early evening heating loads. The idea is to store enough to meet the high demand periods instead of heat being added by a boiler or removed from the system by a cooling tower.

    This is common practice and not in anyway contrary to the laws of thermodynamics.
  25. Sep 8, 2003 #24


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    Whoah! I see now that krab (7 September 10:25 PM post) already covered the thermodynamic formula business. Had not noticed krab's when I wrote this:

    ...does anyone want to comment on a simple formula from thermodynamics that limits the efficiency of AC

    the work (or electricity) cost of pumping a joule of heat from Lo temp to Hi temp
    is always going to be at least

    (H-L)/L joules of work (or electricity)

    in the metric system they prefer for you to measure heat in joules instead of calories or BTU or kWh-----whatever you measure electricity in should also be your measure of heat, that way there are no conversions

    but you could also be more userfriendly and say

    "the work or electricity cost of pumping a kWh of heat from Lo temp to Hi temp is always going to be at least

    (H-L)/L kWh of work or electricity"

    what units doesnt matter as long as consistent.

    So if you keep the inside temp 290 and the outside temp is 310
    (H-L)/L = (310 - 290)/290 = 20/290

    and that is the unavoidable cost in kWh of pumping a kWh of heat from your livingroom and dumping it outdoors

    So this makes it clear what you have to play around with

    If you have a flow of water from the city water main which you are using anyway for something or an underground tank of water or whatever that is cooler than ambient then you can reduce H.

    Since you reduce H you will reduce (H - L)/L which is the energy cost.

    If your AC is efficient it does not seem to matter whether you save energy by reducing the cost of running it by reducing H to H'
    or whether you continue dumping AC heat at H and run a little engine on the side between H and H' to generate some back.

    If you have a flow of cold water to dump heat into (say) then you get the same energy bonus whether you use it to cool the cooling vanes of the AC directly or use it to run an engine that generates power on the temp difference between the vanes of the AC and the cold water heatsink.

    Improving efficiency of the basic unit and using less can be like having an aux generator cranking power on the side. And may be less costly in terms of capital equipment
    Last edited: Sep 8, 2003
  26. Sep 8, 2003 #25


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    Quite right. The rate of heat transfer to the hotter reservoir (usu. the outside air at temp. T2) is proportional to the temperature difference T3-T2, where T3 is the temp. of the working fluid as it comes into the radiator. The a/c unit is working effectively between temps T3 and T1 and so is less efficient than if it were working between T2 and T1. In place of the radiator, you can set up a heat engine working between T3 and T2. But a better scheme would be to reduce T3, and thereby save mechanical energy by making the a/c more efficient. How? Since you still need to radiate the same amount of heat, a lower T3 would require a larger radiator. It's kinda counter-intuitive, I know.

    That's true. But if you already have cold water from the main to cool the air, you don't need an air conditioner; you just need a heat exchanger. Compared to this arrangement, which reqires zero work from compressor, you do need more work from compressor.

    To avoid misunderstanding, let me emphasize I'm talking about what can be done theoretically. You are talking about having a device of fixed characteristics. In that case, of course you can recover some of the wasted heat energy and put it to use. My point is that energy recovery would be more trouble than redesigning your commercial unit to make it more efficient.
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